It is well known that in the last decade there has been enormous growth in the number of wind farms, in Spain as well as abroad. As an example, at the end of 2007 Spain was the third country in terms of installed potency, behind only Germany and the US; with approximately 27,026 GWh of produced energy.
On a global level it is foreseeable that installed potency will increase in a significant manner, in the order of 170 GW in 2010; while in Spain 20,155 MW are contemplated in 2010 and 29,000 MW in 2016.
This type of facility is mainly installed in locations that are very prone to lightning impacts. As a result, in a mere tenth of a second, a bolt of lightning can cause severe damage to a wind turbine.
This damage fundamentally occurs when an atmospheric discharge strikes the blades or paddles of the wind turbine, making it necessary for the wind turbines to have effective protection systems against the impact of lightning.
In general, lightning impacts occur between the months of April and September, with the majority striking in the month of August. Recently, lightning has also been observed to strike in December. Another aspect worth noting is that the number of lightning bolts striking worldwide is increasing. Therefore, if when the counting or measurement of lightning strikes started in 1960 is taken as value 0, in 1997 this value would be around 15 and in 2007 around 100. These values indicate that the number of lightning strikes against the earth have increased on a significant scale.
Therefore, if the number of wind turbines is increasing in a geometric progression and the number of bolts of lightning also, then a higher number of impacts and, potentially, losses, affecting wind farms can also be expected.
Probably the second most important problem faced by wind farms, and, in particular, wind turbines is a bolt of lightning striking any part of it, although more specifically any of the blades. It has been estimated that 20% of wind turbine breakdowns, representing slightly more than 25% of their cost, is due to lightning impacts.
In this regard, bolts of lightning tend to strike at the highest point of a particular zone. For this reason, wind turbines are a natural target given their height in addition to their elevated location. The blades are one of the wind turbine's most expensive components, and a lightning impact can have an extremely destructive effect on an unprotected blade. For this reason, one of the most important problems in terms of blade and wind turbine technology lies in preventing the impact of lightning. Possibly, the impact of lightning and the formation of ice are the most pressing challenges in the field of electric wind power.
Although modern onshore wind turbines are increasingly large, with the ensuing increase in the risk of being struck by lightning, and offshore wind turbines are even more exposed to lightning that onshore ones, all systems however, are fitted with blades, which however large or small they are, are exposed to the impact of lightning at any given time.
Lightning impact can have a highly destructive effect on blades if these are unprotected. Different studies have shown that lightning tends to strike the part furthest away from the root of the blade because it is the highest point. In such cases, an electric arc spreads from the point of contact through other conductive components to the flange connection and can reach a temperature of 30,000° C. The result is an explosive expansion of the air inside the blade. The effects this produces include damage to the surface, pressure damage, delamination, cracks on the selected leading and trailing edges, and the melting of glue. Lightning strikes can also produce hidden damage and cause severe problems in the long term that significantly reduce the useful life of the blade.
An unprotected blade is extremely vulnerable to the impact of lightning. Therefore, to date all wind turbine blades are protected against the impact of lightning. The protection system is based on the principle of Franklin-type rods, a principle that has been discussed previously, albeit with a series of improvements which have included new materials in the lightning strike receptor in such a way that, frequently, the blades can resist the impact of lightning several times before it is necessary to change the materials used in the receptor.
In summary, blade design using current technologies is based on the fact that lightning normally strikes the tip of the blade and must be conducted to earth to be eliminated. The protection system consists of two main components: the receptors located on the surface of the blade and an internal cabling system that conducts the power of the lightning. When lightning strikes, receptors capture it and the cabling system transports the electric charge through the blade to the tower, and from there to earth. Receptors are precisely the conduction points where the lightning strikes the blade.